The present invention is directed to electrochemical gas sensors and, more particularly, to electrochemical gas sensors with a hydrophilic membrane coating.
In electrochemical gas sensors, which are also referred to as gas-selective or gas-sensitive electrodes, the gas molecules to be determined diffuse through a gas-permeable and substantially liquid- and ion-impermeable membrane from a usually aqueous outer solution or even a gas phase into the inner electrolyte space of the sensor. In addition to an inner electrolyte solution, this inner electrolyte space also contains electrodes for the electrochemical determination of the gas and, in particular, measuring electrodes or working electrodes, counterelectrodes or reference electrodes. The actual electrochemical detection reactions for determining the gas occur in this inner electrolyte space usually by means of amperometric or potentiometric methods.
A frequently used gas sensor is for example the oxygen sensor according to Clark. In this case, a gas-permeable membrane separates the inner electrolyte solution from the aqueous outer medium, the measuring medium. In the simplest case two electrodes dip into the inner electrolyte solution, one of which is arranged as a working electrode directly behind the membrane. After a polarization voltage of a suitable magnitude has been applied, the oxygen which has diffused from the measuring medium through the membrane into the inner electrolyte space is reduced at the working electrode and a current flows that corresponds to the turnover. This current is proportional to the partial pressure of oxygen in the measuring medium and is the primary measured quantity. In addition to sensors with two electrodes, those with three electrodes are also commonly used which are operated potentiostatically, whereby the additional electrode assists as a reference electrode in the stabilization of the working point.
Other widespread electrochemical gas sensors which have such gas-permeable membranes are for example sensors of the Severinghaus type for the determination of carbon dioxide or electrochemical sensors for determining hydrogen by means of oxidation on platinum electrodes.
Such electrochemical gas sensors are often used in medical and diagnostic analytical systems to determine gas partial pressures or gas concentrations in liquids. In particular, such electrochemical gas sensors are used in blood gas analyzers which play a major role in diagnostics. Blood gas analyzers often have several sensors connected in series for various parameters. The sample liquid flows through these sensors and the measurement is often carried out in a so-called stop flow procedure in which the sample rests at the time of measurement. Such systems are often used routinely in hospitals, laboratories and doctor's offices and hence high demands are made on their sensors with regard to lifetime, accuracy and reproducibility.
When gaseous analytes in aqueous solutions are determined by means of electrochemical gas sensors, in particular in physiological liquids such as, for example, whole blood, serum or urine, problems can occur in rare cases in the sample measurement or in the calibration or quality control when the sample or the quality control or calibration agent only incompletely fills the sample channel or when gas bubbles such as air bubbles are in the solution in the area of the sensors. Gas bubbles can lead to measurement errors especially in blood gas analytical systems with sensor elements for small sample volumes. Thus, an effective check has to be carried out with regard to the presence or absence of gas bubbles. However, this often requires elaborate and complicated detection methods for detecting gas bubbles in the sensor area such as those described for example in WO 01/33195 (Taagaard et al.). Gas bubbles typically get stuck on the membrane surface. This phenomenon is observed when the aqueous liquid evades the hydrophobic surface of the membrane on one or both sides during the process of filling the sample channel. If the liquid front has the possibility of running past the side of the membrane before the membrane is completely covered with liquid, a gas bubble forms in the area of the membrane. Gas bubbles that are already present as well as those that are newly formed typically remain stuck to the membrane and are often not carried along by a current of liquid. A gas bubble adhering to the membrane or located in the immediate vicinity of the membrane results in a measuring error which cannot be recognized without additional measures for detecting the gas bubble.